This invention relates to imaging systems. More specifically this invention concerns an electrophoretic imaging system.
The use of photoconductors to form images is well known. For example, in xerography such as described in U.S. Pat. No. 2,297,691 to C. F. Carlson, photoconductors are widely used. In this process a layer of a photoconductor on a conductive substrate is first provided with a uniform electrostatic charge on its surface in the dark and is then exposed to a light image which causes the photoconductor to allow charge to dissipate through it to the conductor in light struck areas leaving a pattern of electrostatic charge on the surface of the photoconductor. This electrostatic image is then made visible by any number of methods. In one process finely divided particles of colorant called toner which is attracted by the electrostatic charge are cascaded across the photoconductor. Normally, a carrier material for the toner is used to ensure that all areas of the photoconductor are contacted with toner. The toner material may also be dispersed in an insulating carrier liquid; the combination is called a liquid developer. As liquid developer is brought into contact with the photoconductor the toner material is drawn out of the liquid and is held to the photoconductor by electrostatic attraction. Many variations of the above processes exist. In one variation shown in U.S. Pat. No. 2,892,709 to E. F. Mayer, the surface of the photoconductor is charged through a liquid developer layer while the photoconductor is exposed to a light image. The photoconductor will not accept a charge in illuminated areas thus forming an electrostatic image on dark areas. The toner material in the liquid developer is drawn to or precipitates on the surface of the photoconductor in the charged areas forming a visible image.
The above processes have a number of deficiencies. A major problem is that the photoconductors must be charged to an intitial potential and then discharged in image configuration to provide an electrostatic image of sufficient strength to attract toner particles. These two process steps take a certain amount of time to complete which slows process speed. The process of U.S. Pat. No. 2,892,709 requires that the charging of the photoconductor be accomplished through an insulating liquid having particles dispersed therein which is relatively inefficient. Also, the above processes are subject to background formed by particles adhering to areas of the photoconductor which are not charged. These particles of toner in background areas interfere with final image quality.
Another disadvantage of prior art systems is that the photoconductive layer must be insulating enough to hold a high charge for the time necessary to develop an image. Further, the prior art processes require that the toner material be carefully selected to have the proper triboelectric relationships to the carrier material and to the charge on the surface of the photoconductor. Also, the prior art processes are not capable of producing positive and negative images at the same time and can change image sense only by the addition of complicated process steps.